The Nikon WU-1a and WU-1b are dongles for D3200 and D600 cameras respectively and allow a smartphone app to see the current viewfinder image, snap photos, and share them wirelessly. If you have a different Nikon DSLR you'll need the WT-4 Wireless Transmitter ($1000) or a WT-5A Wireless Transmitter ($877) for the same functionality.

Hardware hacker Joe Fitz figured out a way to make the inexpensive consumer-level WU-1a work with his D800 camera without having to pay for the WT-series wireless transmitters. He accomplished this by hacking the dongle to receive power from the camera using a micro-USB cord instead of a port in the camera itself. See the source link below and the Nikonhacker wiki for full technical details. You won't get the increased range of the WT-series but at less than one-tenth the price that is probably okay for most users.